1. Field of the Invention
The present invention relates to a high-strength high-conductivity copper alloy excellent in fatigue and intermediate temperature characteristics. Particularly, the present invention relates to conductive spring materials used for various terminals, connectors, relays and switches.
2. Description of the Related Art
The following characteristics are required of conductive spring materials used for various terminals, connectors, relays and switches:
(a) sufficient strength for generating high contact pressure even when in the form of thin sheets;
(b) a low stress relaxation ratio with the contact pressure not decreasing after a long-term use at high temperatures;
(c) a high conductivity with small Joule heating, which is generated by a flowing electric current, and radiation of the generated heat;
(d) prevention of crack and roughness formation at bent portions even when applying severe bend processing; and
(e) a high elastic limit so as to enable the alloy to be used under high stress.
Phosphor bronze has been used as a conductive spring material for various terminals, connectors, relays and switches. However, since electronic appliances and components thereof have been required to be small in size and thin, demands for suitable materials have increased accordingly, and improvements in strength, conductivity, heat resistance and fatigue characteristics have been required. Various kinds of Cu—Cr alloys and Cu—Cr—Zr alloys have been developed to comply with these requirements.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 9-087814
Patent Reference 2: Japanese Unexamined Patent Application Publication No. 7-258804
Patent Reference 3: Japanese Unexamined Patent Application Publication No. 7-258806
Patent Reference 4: Japanese Unexamined Patent Application Publication No. 7-258807
Patent Reference 5: Japanese Unexamined Patent Application Publication No. 7-268573
Patent Reference 6: Japanese Patent No. 2682577
The drawability of Cu—Cr alloys decreases at intermediate temperatures around 400° C. While the alloys are not used at temperatures as high as 400° C. in the field of the present invention, and the heat resistance required is around 100° C., or about 200° C. under the most severe conditions required in the present invention, the drawability at intermediate temperatures of around 400° C. is used as a standard of heat resistance. Cu—Cr—Zr alloys have been developed to have improved strength at intermediate temperatures of around 400° C. While Cu—Cr—Zr alloys have excellent fatigue characteristics compared with Cu—Cr alloys, the conductivity of the alloy decreases by increasing the amount of Zr added.
Cu—Cr—Zr alloys are precipitation hardening alloys, and their strength is improved by allowing Cr, Zr or Cu—Zr compounds to precipitate in the copper matrix by aging after solution treatment. However, Cr, Zr or Cr—Zr compounds that are included and which crystallize or precipitate during the casting process remain in the alloy.
Cu—Cr—Zr alloy are usually manufactured by the steps of blending the materials, melting, casting, homogenization annealing, hot rolling, cold rolling if necessary, solution treatment, cold rolling and aging (cold rolling) sequentially applied in this order.
However, the fatigue characteristics are deteriorated in Cu—Cr—Zr alloys since Cu—Zr compounds are readily cleaved by dislocation, and sulfur as one of the inevitable impurities may be concentrated at grain boundaries. The inventors of the present invention found that the grain boundary strength is decreased by concentration of sulfur at the grain boundaries. Accordingly, the object of the present invention is to provide a Cu—Cr—Zr alloy excellent in fatigue and intermediate temperature characteristics.